Field of the Invention and Related Art Statement
The present invention relates to a method of reducing the lateral force variation of a pneumatic tire.
When a tire is rotated, there are produced various kinds of forces. Among these forces is a force generated in a lateral direction termed a lateral force which is sometimes abbreviated as LF. A variation of this lateral force over one revolution is called a lateral force variation which is generally abbreviated as LFV. In order to improve the property of the tire, the lateral force variation has to be reduced.
There have been proposed several methods for reducing LFV of the tire. For instance, in Japanese Patent Publication No. 60-34,452, there is disclosed a method of reducing LFV. In this method, a tire is rotated in one direction and LF values are measured at many points along the circumference of tire and are stored. Then, the tire is rotated in an opposite direction to measure values of LF. Then, algebraic sums of these LF values at corresponding measuring points are derived, and an amount and a position of grinding the tire are determined in accordance with said algebraic sums.
In Japanese Patent Publication No. 61-23,105, there is disclosed another known method, in which a tire is rotated in one direction to measure LF, then a first harmonic is derived from the measured LF waveform, and the tire is ground on the basis of the first harmonic of the measured waveform of LF.
It has been generally recognized that LFV is a composite of the conicity component whose direction is not changed in accordance with the rotational direction of the tire and the plysteer component whose direction is inverse depending upon the rotational direction. In general, the. method of reducing LFV is to grind the tire locally such that the conicity component is reduced.
In the known method described in the above mentioned Japanese Patent Publication No. 60-34,452, the tire is ground in accordance with the sum of the forward and backward lateral forces which represents the conicity component, so that it has been considered that the LFV of the tire could be reduced. However, in practice, this known method could not reduce LFV of the tire efficiently and sometimes LFV of the tire becomes worse by the grinding operation. The inventors have investigated this phenomenon and have found that the tire has inherently a time lag characteristic so that measured waveform of LF is delayed with respect to a factor waveform in the causation of the lateral force. Therefore, it is impossible to derive the conicity component accurately from the algebraic sum of the forward and backward lateral forces and LFV of the tire could not be corrected accurately.
FIG. 1 shows the measured waveforms A and B of the forward and backward lateral forces of the tire. When these LFV values at corresponding points on the tire are summed, the LFV values are canceled out to derive a summed waveform C. Then it is difficult to determine the grinding amount and grinding position of the tire from the waveform C.
Further, in the known method disclosed in the Japanese Patent Publication No. 61-23,105, since the tire is ground in accordance with the LF waveform derived by rotating the tire only in one direction, the LFV might be increased by the grinding. Moreover, also in this known method, the time lag characteristic of the tire is not taken into account.
Recently there has been used a uni-directional tire whose rotational direction is specified or destined. In this uni-directional tire, it is not necessary to separate the lateral force into the conicity and plysteer components and it is sufficient to reduce the lateral force variation when the tire is rotated in the specified direction. Therefore, the method described in the Japanese Patent Publication No. 61-23,105 could be advantageously applied to the uni-directional tire. However, also in this known method, since the time lag of the tire is not taken into account at all, LFV could not be corrected accurately, and sometimes LFV might be increased by grinding the tire.